In the sintering of ore material, the material is mixed with a small-particle carbon-containing solid and placed on a sintering belt, and with at least partial combustion of the solid during the ongoing transport on the sintering belt, the material is transported to a discharge end. Alternatively, the starting material is pelletted or briquetted and subsequently sintered. Combustion air is supplied. During the sintering process, the charging stock passes through a smoldering and, at least partially, a combustion process, by means of which the starting material is agglomerated—that is, sintered—to form larger pieces. Due to the combustion and smoldering process increased by supplied combustion air, considerable gas quantities are released from the charging stock; they contain a whole series of pollutants in addition to the components CO2, perhaps CO, O2, H2O, and/or N2. In particular, they are nitrogen oxides (NOx), SO2, HCl, dioxins, furans, dusts, and sublimatable or condensable residues from the smoldering process, heavy hydrocarbons, and/or heavy metals.
Studies on air maintenance have shown that the waste gases, for example, from sintering belts, contain a very considerable fraction of the total impurities, which are, as a whole, yielded during the production of metal. Thus, in the area of dioxins and furans, contamination fractions of more than 90% of the corresponding emissions were detected in the production of iron and steel. Due to the extraordinarily large waste gas quantities in sintering belt plants, satisfactory gas purification has been possible, up to now, only at very considerable expense, which appreciably increases the cost of steel production as a whole. In particular, as a result of the different fractions of the pollutants in the sintering belt waste gas and its greatly fluctuating composition, which depends on the charging stock, and also because of the very different reactions of the pollutants and the purification methods which are available, it has been necessary to implement a large number of purification steps one after the other.
Thus, for example, so-called airborne flow methods, with downstream filtering-out of the airborne flow particles, and further downstream, catalytic oxidation for dioxin reduction, have been proposed. With these methods, considerable catalyst damage appeared which, in particular, shows up as a surface coating of the catalyst with organic hydrocarbons (Final Report 50 441-5/217, “Reduction of dioxin emissions from sintering plants,” commissioned by the German Federal Environmental Agency, December 2002).
Another waste gas purification method for sintering belt plants has been proposed in WO 01/17663—in such a way that the sintering belt waste gas was purified in an airborne flow-purification stage, with a subsequent adsorption purification stage, wherein in the airborne flow-purification stage, high-value active coke in ground form—that is, with a relatively small particle size—was delivered to the waste gas, in the form of an airborne flow cloud. The finely divided adsorption agent reacts, in the airborne flow phase, with a part of the pollutants to be removed from the sintering belt waste gas. As a post-reaction stage for the airborne flow process, the flying dust was not precipitated, on a cloth filter or an electrofilter however, but rather on the entry side of the counterflow moving bed reactor where the flying dust was precipitated on the particles of the moving bed bulk material—that is, on their surfaces or in the volume between particles. Subsequently, the sintering belt waste gas flowed through the particle layer of the counterflow moving bed reactor, from, for example, active coke, so that the sintering belt waste gas, previously purified in the airborne flow phase, then undergoes an adsorption purification. The airborne flow-purification process, downstream from the moving bed reactor, requires the use of a second particle-shaped purification agent without, in this way, the detrimental catalyst damage in the moving bed already being stopped.
Above all, when removal of the NOx from the sintering waste gas is most important, other pollutants such as SO2 and HCl have proved to be particularly disturbing if the NOx is to be removed from the waste gas with the aid of a catalyst, then these and other pollutants contained in the sintering waste gas are so-called catalyst toxins for the removal of the NOx.
In WO 2006/084671 A1, therefore, a preliminary purification stage for the most extensive removal of, above all, SOx and HCl, and a post-purification stage, a counterflow reactor traversed from above downwards by a catalyst for the deposition of NOx such as a carbon-containing, absorption agent, were used. Here, in a single adsorption and/or absorption middle layer, a two-stage waste gas purification process was carried out in which the first stage was carried out in the entry area, and the second purification stage in the subsequent zones of the adsorption and/or absorption middle layer. It had become evident, namely, that a preliminary purification stage, in which, for example, the SO2 is deposited with calcium hydroxide, was also insufficient, because the residual quantity of SO2 and/or HCl remaining in the waste gas—if it does not come into contact with ammonia, which is required for the NOx conversion—leads to a situation where the catalyst grains of the NOx catalyst, if it is a carbon-containing absorption and/or adsorption agent, such as active coke, can swell thereby (popcorn formation). This effect occurs if ammonium sulfate or ammonium chloride crystals form in the porous catalyst. The expansion of the forming crystals in the pore system breaks the structure of the catalyst. The catalyst, therefore, is not only consumed but also disintegrates. Particle size diminutions in the catalyst bed lead, moreover, to a rise in the pressure loss and, consequently, to another increase in the costs of the purification process. Since, in accordance with WO 2006/084671 A1—the thus damaged catalyst of the lower purification stage of the single reactor bed was, again and again, discharged on the lower tray from the single reactor bed, a sufficient total purification of the sintering belt waste gas was attained. For the preliminary purification of the sintering waste gas before its entry into the moving bed reactor unit, a bag filter or an electrofilter and/or a waste gas scrubber was preferably used. Alternatively or supplementarily, finely distributed reaction and/or absorption agent, such as lime dust and/or active coke dust, could be added to the sintering waste gas in the airborne flow, in order to free the sintering waste gas from at least a part of the pollutants SO2 and HCl before entering into the moving bed reactor plant. Preferably, the previously purified sintering waste gas contained, upon entry into the moving bed reactor plant, an SO2 content of less than 100 mg per standard cubic meter, preferably, no more than 5 mg per standard cubic meter.
What these known preliminary purification stages have in common is that a considerable equipment outlay must be made for this, and additional consumable material, such as lime, must be used and subsequently disposed of or further processed. Moreover, in the case of wet scrubbing methods, corrosion problems, reheating problems, and/or water workup problems have to be overcome.